The present invention relates to a sheet ejection device for stacking sheets to be ejected, on a sheet stacking table, a post-processing apparatus and an image forming system provided with the same.
An image forming system composed of an image forming apparatus, such as a printer, copier, and the like, and post-processing device thereof are mostly equipped with a sheet loading table for temporarily storing plural sheets of paper having images formed thereon within the apparatus or equipped with a sheet ejection tray located outside the apparatus and loaded with the discharged sheet. When the sheet is discharged onto the sheet loading table or the sheet ejection tray (hereinafter, collectively referred to as a sheet stacking table), it happens that the sheet stacking position is deviated during the period after a sheet of paper leaves the sheet conveyance roller (hereinafter, referred to as an ejection roller) located immediately before the sheet stacking table and before the sheet of paper drops by its own weight onto the sheet stacking table. Consequently, there is a problem in that the sheet is not placed on the sheet stacking table in the aligned state. This problem tends to arise regardless of whether the sheet stacking table is horizontally disposed or the sheet stacking table is disposed with the loading surface thereof inclined. Particularly, this problem tends to arise when the sheet conveyance speed is high or environmental conditions are severe (high-temperature and high-humidity condition, or low-temperature and low-humidity condition). Furthermore, it is comparatively easy to align a sheet of paper by an alignment device for aligning the sheet in the direction (sheet width direction) perpendicular to the sheet conveying direction when the sheet of paper is discharged. However, it is difficult to align the sheet of paper in the sheet conveying direction. In order to improve the sheet alignment condition in the sheet conveying direction on the sheet stacking table, a variety of sheet ejection device have been developed. However, there are problems in that the mechanism is complicated and the device becomes too large, or the sheets of paper are not stacked in the aligned state.
To solve the above problems, there is disclosed a technology wherein the rear end of discharged sheet is held by a gripper, and the gripper is moved, thereby placing the sheet onto the sheet ejection tray (for example, see Japanese Patent Application Publication No. 2008-273656). The apparatus of the Japanese Patent Application Publication No. 2008-273656 has a mechanism which stacks, collected sheets in the stacker (sheet stacking table) on the ejecting tray by gripping them. The technology described in Japanese Patent Application Publication No. 2008-273656 is also advantageous because the technology can apply to stapled sheets and shift-processed sheets.
In the technique described in the Japanese Unexamined Patent Application Publication No. 2008-273656, however, it is difficult to align the trailing edges of sheets. Further, when the ejected sheet is received by a gripper, the sheet must be stopped once. A long time is required before sheets are stacked on a sheet stacking table.
In the post-processing apparatus mounted on a normal image forming system, on the other hand, post-processing operations such as binding or punching is often performed on the image-formed sheet coming out of the image forming apparatus. These post-processing operations are interlocked with the operations of the image forming apparatus. This requires a high-speed post-processing apparatus capable of conforming to the processing speed of an image forming apparatus when an image is formed on sheets at a high speed. While the sheets are subjected to post-processing operations such as binding or punching, the post-processing apparatus cannot accept the next sheet at the position (post-processing section) when post-processing operations is performed. To prevent reduction in the productivity of the sheets outputted from the image forming apparatus, the post-processing apparatus is required to accept the sheets without reducing the speed of conveying the sheets outputted from the image forming apparatus, even when post-processing operation is performed.
In one of the post-processing apparatus having been disclosed to solve the above-mentioned problem, the sheet outputted from the image forming apparatus is stopped temporarily on the upstream side of the post-processing section, and the stopped sheet and the next one are placed one on top of the other. After that, these two sheets are fed to the next post-processing section (Japanese Unexamined Patent Application Publication No. 11-157741) for example.
FIG. 13 is an overall schematic diagram of the sheet post-processing apparatus disclosed in the Japanese Unexamined Patent Application Publication No. 11-157741. According to the structure of, the Japanese Unexamined Patent Application Publication No. 11-157741, two sheets are placed one on top of the other by two branch paths 4 and 5 and a stopper located downstream. This document discloses a technique of displacement to ensure that the sheet S2 located on the upper side in the stacking section shifts forward in the direction of ejecting the sheets, when the sheets are aligned in the intermediate stacking section where the overlapped sheets S1 and S2 are temporarily accommodated The stacking section is made up of an end fence 43 and discharge belt 41.
This stacking section (sheet stacking table) is arranged in a slanted position. When the two overlapped sheets are ejected from the sheet ejection roller 24, the sheets are pushed back along the stacking section in the direction opposite to the sheet ejecting direction under its own weight and by a returning roller 45, and are aligned after coming in contact with an end fence 43 located below. When two sheets S1 and S2 are put one on top of the other and are conveyed, misalignment may occur between the two sheets S1 and S2 due to the variations in the diameter of the conveyance roller, the shape of the conveyance path or friction. If the lower sheet (first sheet) S1 shifted forward in the sheet ejecting direction is conveyed to the stacking section before reaching the stacking section, the force of returning the upper sheet by the returning roller 45 cannot be applied to the lower sheet. Thus, this sheet will be accommodated by the stacking section in a misaligned state. To solve this problem, the Japanese Unexamined Patent Application Publication No. 11-157741 provides a forced displacement to ensure that the upper sheet (second sheet) S2 shifts forward in the sheet ejecting direction. The sheet displacement is provided by changing the diameter of the conveyance roller of each branch path or the rotating speed of the conveyance roller, or by arranging stoppers at the different positions of the branch path. In the technique disclosed in the Japanese Unexamined Patent Application Publication No. 11-157741, the sheets are conveyed by being placed one on top of the other, whereby the interval of sheet conveyance is increased and a required post-processing time is provided. Thus, even during the step of post-processing, the sheets outputted from the image forming apparatus can be received by the post-processing apparatus. This arrangement allows the trailing edges of the two sheets S1 and S2 to be aligned on the stacking section.
In the technique of the Japanese Unexamined Patent Application Publication No. 11-157741, however, the mechanism section for displacing the two sheets S1 and S2 and placing one on top of the other is located far from the stacking section. Thus, these two sheets S1 and S2 are shifted before reaching the stacking section, and the original amount of displacement cannot be maintained. Further, when multiple sheets are stacked on the stacking section, the succeeding two sheets may come in contact with the sheet already stacked on the stacking section, and the sheet already stacked on the stacking section may be moved in the sheet ejecting direction.